This invention relates to a unidirectional surface acoustic wave filter and more particularly to a surface acoustic wave filter which can increase the degree of freedom of design with simultaneous attainment of reduction in size.
A unidirectional transducer has hitherto been proposed with a view of reducing insertion loss in a surface acoustic wave filter, as disclosed in, for example, "Low Insertion Loss Acoustic Surface Wave Filter Using Group-type Unidirectional Interdigital Transducer" by Kazuhiko Yamanouchi et al. at Research Institute of Electrical Communication, Tohoku University, Japan, IEEE 1975 Ultrasonics Symposium Proceedings, IEEE (at #75 CHO 994-4SU pp. 317-321, and U.S. Pat. No. 4,422,000 to Jun Yamada and Katashi Hazama entitled "Unidirectional Surface Acoustic Wave Device with Meandering Electrode", patented on Dec. 20, 1983 and assigned to Hitachi, Ltd.
Because of its bidirectionality, an ordinary surface acoustic wave transducer inherently has an insertion loss of 6 dB, even estimated in minimum, whereas a surface acoustic wave transducer having perfect unidirectivity has a loss which is ideally zero.
To explain the operational principle of the unidirectional transducer, reference is now made to FIG. 1 which illustrates a simplified model of an interdigital input transducer and its peripheral circuit. Although not shown, an interdigital output transducer has a similar construction where a load substitutes for a signal source 3 shown in FIG. 1. In FIG. 1, an interdigital sending electrode 5 and an interdigital reflection electrode 6 constitute the interdigital input transducer. The interdigital sending electrode 5 is connected to the signal source 3 through a phase shifter 2 which provides an electrical phase difference of .pi./2 radians. Reference numeral 4 denotes a drive conductance of signal source 3 which is G.sub.l. The interdigital reflection electrode 6 is connected directly to the signal source 3. The distance between the interdigital sending electrode 5 and interdigital reflection electrode 6 is chosen so as to correspond to a geometrical phase difference of .pi./2 radians therebetween. A forward propagation direction and a backward propagation direction are designated by reference numerals 7 and 8, respectively.
In operation, speaking of the forward direction side, surface acoustic waves W.sub.SF and W.sub.RF respectively propagating from the interdigital sending and reflection electrodes 5 and 6 in the forward direction 7 are both delayed, electrically or geometrically, by .pi./2 radians with respect to the phase of a signal of the signal source 3 and consequently in phase with each other at a position of the interdigital sending electrode 5, with the result that a surface acoustic wave propagating from the interdigital input transducer in the forward direction amounts to the sum of W.sub.SF and W.sub.RF. On the backward direction side, a surface acoustic wave W.sub.RR propagating from the interdigital reflection electrode 6 in the backward direction 8 is in phase with the signal of the signal source 3 but a surface acoustic wave W.sub.SR propagating from the interdigital sending electrode 5 in the backward direction 8 is delayed in phase by .pi. radians with respect to the signal of the signal source 3 at a position of the interdigital reflection electrode 6, with the result that the waves W.sub.RR and W.sub.SR are out of phase with each other and cancelled out to nullify a resultant surface acoustic wave propagating in the backward direction 8, thereby attaining the unidirectivity.
To add, since, in the case of the ordinary bidirectional transducer, surface acoustic waves uniformly propagate in the forward and backward directions with respect to the electrode structure or the transducer, there exists an insertion loss of 3 dB, and consequently, taking both the input and output transducers into consideration, there necessarily exists an insertion loss of at least 6 dB as a whole.
In the case of the unidirectional transducer of FIG. 1, both the electrical and geometrical phase differences are set to be .pi./2 radians so as to provide perfect unidirectivity. However, it should be noted that the perfect unidirectivity can be obtained even when the sum of an electrical phase difference due to the phase shifter and a geometrical phase difference between the interdigital sending and reflection electrodes is .pi. radians, as will be known from the disclosure of the aforementioned U.S. Pat. No. 4,422,000.
FIG. 2 shows the construction of a typical prior art unidirectional transducer. In FIG. 2, identical members to those of FIG. 1 are designated by identical reference numerals and will not be detailed. There are seen in FIG. 2, a unidirectional transducer 1, the .pi./2 phase shifter 2, the signal source 3, the drive conductance 4 of the signal source 3 which is G.sub.l, interdigital sending electrodes 5', and interdigital reflection electrodes 6'. Forward and backward directions of propagation of surface acoustic waves are designated by the reference numerals 7 and 8, respectively. The distance between the interdigital sending and reflection electrodes 5' and 6' is set such that the geometrical phase difference therebetween is .pi./2 radians.
With this prior art transducer, unless an input conductance of Ga of the unidirectional transducer (inclusive of a conductance of the .pi./2 phase shifter) as viewed from the signal source side substantially equals the drive conductance G.sub.l of the signal source, it is impossible to suppress undesired transducer reflection and to keep an insertion loss low. Therefore, restrictions are imposed on size and shape of the unidirectional transducer and the value of the drive conductance, resulting in such inconvenience as impairment of the degree of freedom of design. More particularly, when making an attempt to substantially match the input conductance of the transducer with a large drive conductance of the signal source, the transducer aperture length W shown in FIG. 2, must be increased and consequently, the device chip is increased in size to raise manufacture cost. Also, when the unidirectional transducer is employed in mass-production products such as, for example, intermediate frequency filters in color television receiver sets, the following problems arise. That is, a signal source conductance (for example, an output conductance of a tuner) and the input conductance of the unidirectional transducer vary about respective predetermined center values due to the variations in such as values of parts constituting the .pi./2 phase shifter, manufacture processes of the tuner and the .pi./2 phase shifter, and the like. Accordingly, to adjust the conductance of signal source or the input conductance of unidirectional transducer, it is necessary either to insert a variable conductance element in the signal source line or to effect adjustment of circuit constants of the phase shifter or trimming of the transducer. Such inconveniences also occur in the load conductance and the output conductance of the unidirectional transducer and therefore, any of the adjustments described above are required.